In recent years, there has been a demand for an increase in the amount of transferable data in mobile communication. Furthermore, for mobile communication, there has been a demand to realize a multimedia sever. Thus, radio scheduling requires control such that different types of QoS (Quality of Service) are ensured for the respective services and such that the throughput of the system is maximized. Radio scheduling as described herein is to assign a radio resource for the communication between a radio base station and a plurality of mobile stations present in the area of the radio base station.
For example, IEEE 802.16-2004, which is a radio communication standard, defines UGS (Unsolicited Grant Service), rtPS (Real-time Polling Service), nrtPS (Non real-time Polling Service), and BE (Best Effort) as QoS service types. The QoS service types involve different types of requested service quality (QoS). The QoS is, for example, the quality of delay or transmission rate.
The UGS is a service for transferring data of a fixed data rate in real time. The rtPS is a service for transferring data of a variable data rate in real time. The nrtPS is a non-real-time service requiring an ensured given band. The BE is a service requiring no ensured given band.
JP2007-28638A (hereinafter referred to as “Document 1”) proposes that with a variation in QoS depending on the QoS service type taken into account, data be sorted into QoS classes in accordance with requested QoS and scheduling be performed on each of the QoS classes. This scheduling needs to allow limited radio resources to be efficiently utilized while meeting the QoS request for each service.
Main known scheduling methods include a round robin method, a PF (Proportional Fairness) method, and a Maximum CINR (Carrier-to-Interference-and Noise Ratio).
The round robin method fairly assigns radio resources to the plurality of mobile stations with radio quality not taken into account. The PF (Proportional Fairness) method assigns the radio resources to the mobile stations with radio quality between the radio base station and each mobile station taken into account. The Maximum CINR (Carrier-to-Interference-and Noise Ratio) also takes radio quality into account. Examples of radio quality as described herein include the CINR and SINR (Signal to Interference and Noise Ratio).
FIG. 1 is a diagram illustrating scheduling in accordance with the round robin method. As seen in FIG. 1, the round robin method sequentially assigns radio resources to a plurality of mobile stations belonging to the same radio base station. The radio resource is fairly assigned to the respective mobile stations.
In contrast, the PF method and the Maximum CINR method perform scheduling with radio quality taken into account, thus serving to improve the system throughput compared to the round robin method. When the PF method or the Maximum CINR method is used, an AMC (Adaptive Modulation Coding) technique is also used. In the AMC, a modulation scheme or a coding rate used for each mobile station is selected in accordance with radio quality. A modulation scheme or a coding rate with a high data transmission rate is selected for a mobile station having high radio quality. A modulation scheme or a coding rate with a low data transmission rate is selected for a mobile station having low radio quality. This control allows a higher system throughput to be accomplished according to the PF method or the Maximum CINR method than according to the round robin method.
The processing in the PF method and the Maximum CINR method will be described.
First, the PF method will be described.
In the PF method, with fairness among the plurality of mobile stations taken into account, the radio resources are assigned to the mobile stations in accordance with the instantaneous radio quality of each mobile station. Here, an index for the mobile station is defined as (k). Furthermore, the instantaneous radio quality of the mobile station (k) is defined as γk, and the average radio quality of the mobile station (k) is defined as γave,k. 
First, Expression (1) is used to calculate the value of an evaluation function Ck for each mobile station. Then, a radio resource is assigned to a mobile station having an evaluation function Ck with a large value.
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                      C          k                =                              γ            k                                γ                          ave              ,              k                                                          (        1        )            
In the PF method, a radio resource is assigned to a mobile station having high instantaneous radio quality with respect to the average radio quality. High radio quality allows the data transmission rate to be increased according to the AMC. Assigning a radio resource to a mobile station having high instantaneous radio quality means assigning a radio resource to a mobile station that has a high data transmission rate. Thus, the system throughput is improved. Furthermore, the PF method uses the instantaneous radio quality with respect to the average radio quality to maintain high fairness among the mobile stations.
Now, the Maximum CINR method will be described.
In the Maximum CINR method, a radio resource is assigned to a mobile station having a high CINR, which is indicative of instantaneous radio quality. First, Expression (2) is used to calculate the value of the evaluation function Ck for each mobile station. Then, a radio resource is assigned to a mobile station having an evaluation function Ck with a large value.[Expression 2]Ck=γk  (2)
In the Maximum CINR method, a radio resource is assigned to a mobile station having high instantaneous radio quality. Thus, the Maximum CINR method further improves the system throughput compared to the PF method. However, compared to the PF method, the Maximum CINR method reduces opportunities to assign a radio resource to a mobile station having low average radio quality. Hence, the PF method offers higher fairness than the Maximum CINR method.
For example, the tendency is to increase opportunities to assign a radio resource to a mobile station located near the radio base station and to offer high radio quality, while reducing opportunities to assign a radio resource to a mobile station located far from the radio base station and to offer low radio quality. As a result, a state is likely to occur in which mobile stations located farther from the radio base station are more likely to fail to maintain demanded QoS.